Send x-opencode-session header on API requests (required by OpenCode Go from 09/05)

Hi — we operate the OpenCode Go managed-inference API, and 280 of your users' orgs call it through MiMoCode (UA mimocode/0.1.13 ai-sdk/provider-utils/4.0.23 runtime/bun/1.3.14).

Starting 09/05, requests without an x-opencode-session header will error on our side — we need a stable per-conversation ID for routing/optimization. MiMoCode doesn't send it in any version we've seen.

Ask: include x-opencode-session: <stable-id-per-conversation> on all outbound inference HTTP requests. Any stable UUID per conversation works. Happy to confirm from our telemetry once a build ships it. Thanks!